As is known, integrated circuits are used in a wide variety of electronic equipment, including portable, or handheld, devices. Such handheld devices include personal digital assistants (PDA), CD players, MP3 players, DVD players, AM/FM radio, a pager, cellular telephones, computer memory extension (commonly referred to as a thumb drive), etc.
These handheld devices include one or more integrated circuits to provide the functionality of the device. For example, a thumb drive may include an integrated circuit for interfacing with a computer (for example, personal computer, laptop, server, workstation, etc.) via one of the ports of the computer (for example, Universal Serial Bus, parallel port, etc.) and at least one other memory integrated circuit (for example, flash memory). As such, when the thumb drive is coupled to a computer, data can be read from and written to the memory of the thumb drive. Accordingly, a user may store personalized information (for example, presentations, Internet access account information, etc.) on his/her thumb drive and use any computer to access the information.
Standards interface specifications provide additional device functionally by facilitating interoperability of handheld devices with host or peripheral devices. For instance, devices may be interoperably coupled as host-to-host, as host-to-peripheral, and/or as peripheral-to-peripheral.
Apart from communication interoperability, some standards interface specifications allow peripheral or handheld devices to draw at least a standby current from a host when connected thereto. For example, when a handheld MP3 player is coupled to a host computer via a USB (Universal Serial Bus) connection, the MP3 may be powered via a 5-volt source of the USB connection instead of using its battery power source. In a normal power mode, a peripheral device (for example, an MP3 player) may draw up to a 100 mA current from the 5-volt source for power. At least one version of a USB specification, however, has general interface parameters that call for limiting the current that a peripheral device may draw to a very small amount (for example, a standby current of less than 1 mA). Accordingly, a peripheral device, such as an MP3 player, should be capable of being self powered (that is, from a battery power source), being powered from a USB connection in a normal power mode, and being powered from a USB connection in a standby current mode.
Therefore, a need exists for power circuitry that accommodates multiple states for battery and non-battery power sources of a dual-power capable device such that the functionality of a peripheral or handheld device is maintained.